In a conventional cellular type communication system, a communication unit such as a portable or mobile radio can move from one geographic cell site location to another. In order to maintain the communications link when the communication unit travels outside of the coverage area of the currently registerd cell site, the communication system must hand off the call to a new cell site. Otherwise, calls may be terminated prematurely.
Conventional cellular systems typically have a central controller that instructs the communication unit as to which cell site it should move to in order to access another cell site for reliable communication. The central controller determines which cell site has the best communication quality with the communication unit by using a voting arrangement where different cell site receivers report on the strength or quality of signal received from the communication unit. In these cellular systems, the central controller monitors each call from a communication unit at its assigned site as well as at cell sites in the adjacent cells. The measured signal quality (that would indicate whether a hand off would be necessary for continued communications) are relayed to a switching center by way of telephone lines to each cell site. When the signal characteristics indicate that a hand off is required, a suitable new cell site is chosen, and a hand off message is sent to the communication unit. This type of hand off is completely managed by the fixed network equipment, and does not require any input from the individual communication units. While such an approach provides for an effective hand-off, it requires a substantially complex and expensive infrastructure and fails to consider the signal quality at the mobile unit.
In another approach where the communication unit it self is involved in the hand off process, the communication unit determines when and which cell site to hand off to by perodically scanning the adjacent cell sites. The scanning process may briefly interrupt the audio signal present in a communication unit call. However in this approach, if the scanning is done too often, serious degradation of the audio quality would occur. On the other hand, if the scanning is done too infrequently, the communication unit may lose a call due to a rapid degradation in the signal quality.